The present invention relates to an improved tube structure, particularly a tube structure designed to be chiefly subject to bending stress and required to be as light as possible.
More specifically, the present invention relates to a tube for bicycle and motorcycle frames having improved characteristics, especially suited for forming frames of competition bicycles with highly distinctive features.
As known, various types of tubes--easy to be found on the present market--have been made available by technique for the forming of bicycle and motorcycle frames. While, for the more common constructions, use is made of simple ordinary cylindrical tubes of uniform thickness, when special frames are constructed--wherein a great lightness has to be combined with a high rigidity--the following types of tubes are currently used: (a) special steel tubes with reduced uniform thicknesses; (b) special steel tubes with longitudinally variable thicknesses; (c) light alloy tubes with uniform thickness; (d) light alloy tubes of uniform thickness, with reinforcements in specific sections applied by glueing inserts.
In all these types of tubes, each straight cross section has a uniform thickness of the tube wall.
It is evident that, in order to improve the characteristics of these tubes, it is necessary to simultaneously increase their stiffness and lightness, that is, to work on two characteristics which are inversely proportional one in respect to the other: in fact, an increase of the lightness is usually prejudicial to the stiffness, both if obtained by acting on the proportioning of the parts and if obtained through choice of the materials. Modern metallurgy and the expedients adopted in proportioning and planning the structures have allowed, in recent years, to achieve important progresses, but the requirements are increasingly pressing particularly in frames for competition bicycles, wherein stiffness is an essential element for a proper and lasting response of the machine to the athlete's requirements--increasingly tending to reach border-line results--and wherein lightness even forms part of the factors which contribute to said border-line results, extolling the performances of the athlete-machine binomial. In the case of motorcycles, the requirements are perhaps less sophisticated, but nonetheless felt, in view of the constructive and energetic problems wherein lightness and stiffness play an important part, particularly from the economical point of view.
In facing the aforementioned problems for the setting up of an improved tube structure for bicycle and motorcycle frames, the Applicant has discovered that, in said frames, the traditional structure of the manufactured article and the components forming said article are apt to guarantee a high and sufficient stiffness on the plane (vertically and longitudinally to the running direction of the vehicle) of said frame, whereas transversally to said plane the stiffness never reaches high values. On the other hand, particularly in bicycle frames, the transversal stiffness is of considerable importance, since the effort made by the cyclist, as well as moving the bicycle forward also turns into a torque tending to cause a transversal flexure in the frame.
Thus, the more flexible the transversal plane of the frame, the higher the cyclist's efforts being wasted, due to said passive torque, to the detriment of a successful performance. In short, the higher the transversal flexure, the lower the cyclist's performance. It thus appears evident that it is imperative for competition bicycles, and appropriate for normal bicycles and motorcycles, to be equipped with frames having a high transversal stiffness and therefore with tubes apt to guarantee such high transversal stiffness to the frames, without losing in lightness.
In view of the aforespecified problems and requirements, the object of the present invention is an improved tube structure designed to be chiefly subject to bending stress and especially suited for forming frames of bicycles and motorcycles with highly distinctive features.